A variety of processes contact finely divided particulate material with a hydrocarbon containing feed under conditions wherein a fluid maintains the particles in a fluidized condition to effect transport of the solid particles to different stages of the process. Fluid catalytic cracking (FCC) is a prime example of such a process that contacts hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The hydrocarbon feed fluidizes the catalyst and typically transports it in a riser as the catalyst promotes the cracking reaction. As the cracking reaction proceeds, substantial amounts of hydrocarbon, called coke, are deposited on the catalyst.
A high temperature regeneration, typically within a regeneration zone, burns coke from the catalyst by contacting the catalyst with an oxygen-containing stream that again serves as a fluidization medium. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by various separation methods within the reaction zone such as ballistic and/or centrifugal separation. However, the catalyst particles employed in FCC processes typically have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials may retain hydrocarbons within their pores, upon the external surface of the catalyst, as well as within the spaces between individual catalyst particles. Although the quantity of hydrocarbons retained on each individual catalyst particle may be very small, the vast amount of catalyst and the high catalyst circulation rate which is typically used in modern FCC processes results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common to remove, or strip, hydrocarbons from spent catalyst prior to passing it into a regeneration zone. Improved stripping brings economic benefits to the FCC process by reducing “delta coke.” Delta coke is the weight percent of the coke on spent catalyst less the weight percent of the coke on regenerated catalyst. Reducing delta coke in the FCC process causes a lowering of the regenerator temperature. Consequently, more of the resulting, relatively cooler regenerated catalyst is required to supply the fixed heat load in the reaction zone. The reaction zone may hence operate at a higher catalyst-to-feed or catalyst-to-oil (C/O) ratio. The higher C/O ratio increases conversion which increases the production of valuable products. Thus, improved stripping results in improved conversion.
The most common method of stripping hydrocarbons from the catalyst utilizes a stripping gas, usually steam, passed through a stream of catalyst, counter-current to the direction of flow of the catalyst. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and adsorbed on the catalyst.
Various prior art designs for stripping vessels are disclosed in U.S. Pat. Nos. 4,481,103, 4,364,905, 5,716,585, 6,680,030, and 6,224,833. A more modern and improved stripping vessel is disclosed in U.S. Pat. No. 7,332,132 which utilizes a structured packing section that comprises a plurality of ribbons. More specifically, the ribbons comprise angular bends and openings between adjacent edges to allow catalyst to flow uniformly into a stripping vessel with relatively small occasion of clogging by spalling coke deposits.
Often a refiner or processor may utilized newer designs for internal equipment within existing stripping vessels. The use of the existing stripping vessels allows the refiner or processor to minimize capital expenditures while utilizing improvements in the refining and processing industry to increase conversions and yields. For example, it is possible to include the newer and more efficient structured packing sections in existing FCC stripping vessels.
In existing FCC stripping vessels which have been reconfigured to include structured packing, since the structured packing may occupy less vertical space within the vessel, there may be a large distance between the structure packing and the inlet to the stripping section. If unobstructed, the catalyst level above the packing can sometime be excessive, greater than 0.91 m (3 ft), and in some cases as much as 4.5 m (15 ft) or greater. If this space is left devoid of equipment, the catalyst may accumulate which requires a refiner to maintain a higher catalyst inventory.
Additionally, the over accumulation of catalyst can result in catalyst compression leading to gas bypassing where the rising steam will channel through the stripping vessel without entering into an emulsion phase with the catalyst to remove the entrained hydrocarbons. This leads to a reduction in stripping efficiency and hydraulic issues in, for example, the reactor, the stripping vessel, and the spent catalyst standpipe.
Therefore, there remains a need for an effective and efficient design for a stripping vessel that includes structured packing and minimizes the risk of channeling and mal-distribution of the catalyst and stripping medium vapors.